MTSL (S-(1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)methyl methanesulfonothioate) is an organosulfur compound that is used as a nitroxide spin label. MTSL is bifunctional, consisting of the nitroxide and the thiosulfonate ester functional groups. The nitroxide label is sterically protected, so it is relatively unreactive.